1. Field of the Invention
The present invention relates generally to a refrigerating apparatus such as a refrigerator, ice making machine, cooled showcase or the like in which a defrosting and/or deicing operation is carried out by the use of hot gas. More particularly, the present invention is concerned with a method of protecting a refrigerating apparatus at the time when hot gas is flowing through a hot gas bypass pipeline incorporated in the refrigerating apparatus.
2. Description of the Prior Art
Heretofore, various proposals have been made in conjunction with the protection of a refrigerating circuit or refrigerating unit of a refrigerating apparatus. As a typical one of such proposals, a protection apparatus is disclosed in Japanese Patent Application Laid-Open No. 162571/1980 (JP-A-55-162571). In this known protection apparatus, the temperature of the casing of a compressor is detected for producing an abnormality alarm or for stopping the operation of the compressor when the detected temperature has attained a predetermined value, so as to prevent the compressor from being overheated due to a decrease of the coolant in the refrigerating unit brought about by leakage thereof. The hitherto known protection apparatus including the abovementioned type can accomplish the intended function in a satisfactory manner so far as the protection of the compressor is concerned, which is so to speak, the heart of the refrigerating unit. In recent years, however, there has been a tendency to increasingly use synthetic resin as the material constituting various parts of the refrigerating unit, whose ability to withstand heat is poor when compared to metal. In this connection, it must be noted that the heretofore known protection apparatus can not ensure adequate protection for parts made of synthetic resin.
For a better understanding of the present invention, description will be made of the problems of the prior art in more concrete terms by referring to FIG. 1 of the accompanying drawings. As is well known, in the course of operation of a refrigerating apparatus of the type mentioned above, a normal refrigerating cycle repetitively alternates with a defrosting cycle in which a hot gas produced by the compressor is directly introduced into an evaporator by way of a hot gas bypass pipeline for periodically removing frost and ice deposited on the evaporator. In FIG. 1, a curve T.sub.a represents changes in the temperature of an intake pipe of the compressor during the course of the repetitive cycles mentioned above, and a curve T.sub.b represents changes in the temperature of the casing of a compressor. As is readily understood by those skilled in the art, the intake pipe temperature T.sub.a of the compressor is lowered over time t during the refrigerating cycle during normal operation, while the compressor casing temperature T.sub.b is increased. On the other hand, during the defrosting cycle, the compressor intake pipe temperature T.sub.a increases over time with the compressor case temperature T.sub.b being decreased.
A solenoid valve is installed in the hot gas bypass pipeline. By closing the solenoid valve, the operation of the refrigerating apparatus is changed over from the defrosting cycle mode to the refrigerating cycle mode. Under certain circumstances, when a failure occurs in a control circuit for controlling the opening/closing operation of the solenoid valve or when an abnormality such as jamming occurs in the solenoid valve itself due to foreign material or particles, the solenoid valve may remain unclosed thereby causing the refrigerating apparatus to abnormally continue the defrosting cycle, whereby the hot gas continues to flow into the evaporator, resulting in the temperature T.sub.a on the low-pressure side of the refrigerating unit (i.e. the compressor intake pipe temperature) steeply increasing, accompanied by a rapid increase in the temperature of the evaporator.
The temperature T.sub.b of the compressor casing tends to increase beyond the temperature T.sub.a of the compressor intake pipe under the influence of heat generated by the compressor driving motor as well as the heat carried by the exhaust gas. The highest temperature that the casing of the reciprocating compressor can withstand is usually to about 80.degree. C. Accordingly, the temperature T.sub.b1 at which the temperature detecting switch for detecting the temperature T.sub.b of the compressor casing can respond for protecting the refrigerating circuit is usually set at a value not higher than 80.degree. C. Accordingly, when the solenoid valve is prevented from closing for the reasons mentioned above to thereby allow the hot gas to continue to flow into the evaporator, the temperature T.sub.a of the compressor intake conduit will rise to a level T.sub.a1 (about 70.degree. C.) shown in FIG. 1 at the time when the temperature detecting switch can respond to the increased temperature T.sub.b1 of the compressor casing, resulting in the temperature of the evaporator becoming higher than the temperature T.sub.a1, as has been confirmed experimentally. Needless to say, the evaporator is the cooling source for the refrigerator as well as the ice making machine and is installed within a housing. Consequently, if the evaporator is heated to a high temperature, those parts made of thermoplastic resin materials such as, for example, ABS resin, vinyl chloride or the like and which are disposed in the vicinity of the evaporator, such as typified by the inner fittings of a refrigerator or the water tank of an ice maker, may undergo thermal deformation or melting in extreme cases, resulting in fatal damage to the refrigerating apparatus, even when no abnormality takes place in the refrigerating unit. In the worst case, the above-described phenomenon may lead to a fire.
In order to prevent such accidents by resorting to the use of the protection apparatus described above, it is necessary that the protection apparatus be able to operate without fail before the evaporator of the refrigerating unit installed at the low-pressure side of the compressor has been heated to the dangerously high temperature mentioned above. To this end, the temperature at which the temperature detecting switch mounted on the casing of the compressor can respond has to be set at a low level such as, for example, T.sub.b2 rather than T.sub.b1. In that case, however, the function of the protection apparatus may be triggered even when the refrigerating circuit operates normally, giving rise to problems with respect to the reliability of the protection apparatus and degradation in the operation efficiency of the refrigeration apparatus.
In this way, the hitherto known protection mechanism operating based on the detected temperature of the compressor casing is incapable of dealing with abnormal temperature rises occurring at the low-pressure side of the refrigerating unit when the hot gas defrosting cycle is extended for some reason, without involving additional problems.